Italy's Supreme Court judges have said that the three suspects in the murder of Meredith Kercher in Perugia last November must remain in prison because there are "grave indications" of their guilt and they could flee if released.

Outlining their motives for refusing an appeal by the three - Amanda Knox, Raffaele Sollecito and Rudy Hermann Guede - against their continued detention earlier this month, the panel of Court of Cassation judges said that Ms Knox, an American from Seattle, had a "negative personality".

They said that Mr Sollecito, her former Italian boyfriend, had a "fragile character" which could not be explained away by "innocuous youthful stereotypes" because of "a context marked by habitual drug use". If released, all three might try to flee or to interfere with evidence, the judges ruled.

At a hearing in Perugia on Saturday before Claudia Matteini, the investigating judge, three court-appointed forensic science experts said there was not enough evidence to show that Ms Kercher had been sexually assaulted, as police claim. There were signs that she had been engaged in sexual activity before being murdered but it was not clear if the sex had been consensual, the experts said.

They also said that although their tests showed that there appeared to be high alcohol levels in her blood, this was misleading because the blood had been contaminated since the original post-mortem examination. There was, therefore, no evidence that she was drunk when killed, nor was there was any trace of drugs in her blood. Investigators maintain that Ms Kercher was killed in a drugs-fuelled sex game that went wrong.

Ms Kercher, a Leeds University exchange student, was found semi-naked under a duvet with her throat cut on November 2 last year in her bedroom in the rented whitewashed hillside cottage in Perugia that she shared with Ms Knox and two women Italian students. Of the three suspects, only Mr Guede admits that he was at the cottage on the evening of the murder but he denies any involvement in the killing.

He has said that he had consensual sex with Ms Kercher but was in the bathroom with a stomach complaint when the killer or killers entered. He recently identified Mr Sollecito and Ms Knox as the people he says he saw at the cottage, claiming that he struggled with Mr Sollecito, who was carrying a knife. Both Mr Sollecito and Ms Knox deny they were at the cottage, claiming that they were both at Mr Sollecito's flat, where they spent the evening smoking cannabis.

The forensic science experts said a kitchen knife found at Mr Sollecito's flat with the DNA of both Ms Knox and Ms Kercher on it was "not incompatible" with the wounds to Ms Kercher's throat. It could also have been used to cut the strap of Ms Kercher's bra.

Italian media reports said Giancarlo Umani Ronchi, head of the panel of the three experts, told the judge his reconstruction was that Ms Kercher had been threatened several times with a knife, "immobilised, and then killed". Defence lawyers complained to the judge that this was supposition.

Ms Kercher's mother Arline, her brother Lyle and sister Stephanie flew from England to attend the hearing. Lyle Kercher said: "Despite attempts to perhaps discredit the evidence and undermine the process, we have every faith in the police and in the forensic experts, our legal team and the Italian legal system".

In what appears to be an orchestrated media campaign defence lawyers have suggested that forensic procedures were not followed properly and that evidence collected from fingerprints, footprints and a fragment of Ms Kercher's bra strap had been contaminated to the point where it would not stand up in court if charges were brought.

Of the three suspects only Mr Sollecito - who has grown his hair long while in prison - attended the hearing. He and the Kercher family did not communicate. Mrs Kercher and Stephanie Kercher left the courtroom when official video footage and photographs of police scientists examining Ms Kercher's bloodied corpse were shown.

Mr Kercher said a decision by a private Italian television channel to broadcast the images had been "in poor taste and unnecessary". ANSA, the Italian news agency, said the family was to sue both the television station in Bari which broadcast the footage and an Italian magazine which had reported that Ms Kercher was killed while in an "alcoholic stupor".

Vicenza Liviero, another of the panel of experts at the hearing, told reporters that Ms Kercher had died from choking and bleeding but it had not been possible to determine the precise time of death. However Francesco Maresca, the Kercher family's Italian lawyer, said there was a consensus that Ms Kercher died between 10pm and midnight.

The family said in a statement: "Almost six months since she died we are still coming to terms with the idea of never seeing our Meredith smiling and happy again. We are here today and it is easier to see the motive as to why Meredith fell in love with Perugia and why she chose to study Italian here, with the prospect of maybe teaching or working in politics. Undoubtedly she would have chosen a career in which she would have made the difference."

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